The sites of degradation of purified rat low density lipoprotein and high density lipoprotein in the rat

Low density lipoprotein and high density lipoprotein were isolated from rat serum by sequential ultracentrifugation in the density intervals 1.025-1.050 g/ml and 1.125-1.21 g/ml, respectively. The isolated lipoproteins were radioiodinated using ICl. Low density lipoprotein was further purified by concanavalin A affinity chromatography and concentrated by ultracentrifugation. 95% of the purified low density lipoprotein radioactivity was precipitable by tetramethylurea, while only 4% was associated with lipids. The radioiodinated high density lipoprotein was incubated for 1 h at 4 degrees C with unlabelled very low density lipoprotein, followed by reisolation by sequential ultracentrifugation. Only 3% of the radioactivity was associated with lipids and 90% was present on apolipoprotein A-I. The serum decay curves of labelled and subsequently purified rat low and high density lipoprotein, measured over a period of 28 h, clearly exhibited more than one component, in contrast to the monoexponential decay curves of iodinated human low density lipoprotein. The decay curves were not affected by the methods used to purify the LDL and HDL preparations. The catabolic sites of the labelled rat lipoproteins were analyzed in vivo using leupeptin-treated rats. In vivo treatment of rats with leupeptin did not affect the rate of disappearance from serum of intravenously injected labelled rat low density lipoprotein and high density lipoprotein. Leupeptin-dependent accumulation of radioiodine occurred almost exclusively in the liver after intravenous injection of iodinated low density lipoprotein, while both the liver and the kidneys showed leupeptin-dependent accumulation of radioactivity after injection of iodinated high density lipoprotein.     

Specific saturable binding of rat high-density lipoproteins to rat kidney membranes

The binding of rat 125I-labelled high-density lipoprotein (HDL) to rat kidney membranes was studied using HDL fractions varying in their apolipoprotein E content. The apolipoprotein E/apolipoprotein A-I ratio (g/g) in the HDL fractions ranged from essentially 0 to 1.5. All these HDL preparations showed the same binding characteristics. The saturation curves, measured at 0 degrees C in the presence of 2% bovine serum albumin, consisted of two components: low-affinity non-saturable binding and high-affinity binding (Kd about 40 micrograms of HDL protein/ml). Scatchard analyses of the high-affinity binding suggest a single class of non-interacting binding sites. These sites could be purified together with the plasma membrane marker enzyme 5'-nucleotidase. The binding of rat HDL to rat kidney membranes was not sensitive to high concentrations of EDTA, relatively insensitive to pronase treatment and influenced by temperature. The specific binding of rat HDL was highest at acid pH and showed an additional optimum at pH 7.5. On a total protein basis unlabelled rat VLDL competed as effectively as unlabelled rat HDL for binding of 125I-labelled rat HDL to partially purified kidney membranes. Rat LDL, purified by chromatography on concanavalin A columns and human LDL did not compete. Unlabelled human HDL was a much weaker competitor than unlabelled rat HDL and the maximal specific binding of 125I-labelled human HDL was only 10% of the value for 125I-labelled rat HDL.     

Characterization of Na+-dependent phosphate uptake in cultured kidney cells (JTC-12) from monkey.

O-(4-Diazo-3-[125I]iodobenzoyl)sucrose ([125I]DIBS), a novel labelling compound specifically designed to study the catabolic sites of serum proteins [De Jong, Bouma, & Gruber (1981) Biochem. J. 198, 45-51], was applied to study the tissue sites of degradation of serum lipoproteins. [125I]DIBS-labelled apolipoproteins (apo) E and A-I, added in tracer amounts to rat serum, associate with high-density lipoproteins (HDL) just like conventionally iodinated apo E and A-I. No difference is observed between the serum decays of chromatographically isolated [125I]DIBS-labelled and conventionally iodinated HDL labelled specifically in either apo E or apo A-I. When these specifically labelled HDLs are injected into fasted rats, a substantial [125I]DIBS-dependent 125I accumulation occurs in the kidneys and in the liver. No [125I]DIBS-dependent accumulation is observed in the kidneys after injection of labelled asialofetuin or human low-density lipoprotein. It is concluded that the kidneys and the liver are important sites of catabolism of rat HDL apo E and A-I.     

Recent studies of lipoprotein kinetics in the metabolic syndrome and related disorders

PURPOSE OF REVIEW: Dyslipoproteinemia is a cardinal feature of the metabolic syndrome that accelerates atherosclerosis. Recent in-vivo kinetic studies of dyslipidemia in the metabolic syndrome are reviewed here. RECENT FINDINGS: The dysregulation of lipoprotein metabolism may be caused by a combination of overproduction of VLDL apolipoprotein B-100, decreased catabolism of apolipoprotein B-containing particles, and increased catabolism of HDL apolipoprotein A-I particles. Nutritional modifications and increased physical exercise may favourably alter lipoprotein transport by collectively decreasing the hepatic secretion of VLDL apolipoprotein B and the catabolism of HDL apolipoprotein A-I, as well as by increasing the clearance of LDL apolipoprotein B. Conventional and new pharmacological treatments, such as statins, fibrates and cholesteryl ester transfer protein inhibitors, can also correct dyslipidemia by several mechanisms, including decreased secretion and increased catabolism of apolipoprotein B, as well as increased secretion and decreased catabolism of apolipoprotein A-I. SUMMARY: Kinetic studies provide a mechanistic insight into the dysregulation and therapy of lipid and lipoprotein disorders. Future research mandates the development of new tracer methodologies with practicable in-vivo protocols for investigating fatty acid turnover, macrophage reverse cholesterol transport, cholesterol transport in plasma, corporeal cholesterol balance, and the turnover of several subpopulations of HDL particles.     

Studies on the metabolism of rat serum very low density apolipoprotein.

There was a rapid transfer of radioactive peptides to other lipoprotein fractions during the first 30 min after the intravenous injection of 125I-labeled rat very low density lipoprotein (VLDL) into rats. After this initial redistribution of radioactivity, label disappeared slowly from all lipoprotein fractions. The disappearance of 125I-labeled human VLDL injected into rats was the same as that of rat VLDL. Most of the radioactivity transferred from VLDL to low density (LDL) and high density (HDL) lipoproteins was associated with two peptides, identified in these studies by polyacrylamide gel electrophoresis as zone IVa and IVb peptides (fast-migrating peptides, possibly analogous to some human C apolipoproteins), although radioactivity initially associated with zone I (analogous to human apolipoprotein B) and zone III (not characterized) was also transferred to LDL and HDL. That the transfer of label from VLDL to LDL and HDL primarily involved small molecular weight peptides was confirmed in studies using VLDL predominantly labeled in these peptides by in vitro transfer from 125I-labeled HDL. Both zone I and zone IV radioactivity was rapidly removed from VLDL during the first 5 min after injection. However, although most of the zone IV radioactivity was recovered in LDL and HDL, only 12% of the label lost from zone I of VLDL was recovered in other lipoproteins, with the remainder presumably having been cleared from the plasma compartment. We have concluded that, during catabolism of rat VLDL apoprotein, there is a rapid transfer of small molecular weight peptides to both LDL and HDL. During the catabolic process, most of the VLDL is rapidly removed from the circulation, with only a small portion being transformed into LDL molecules.     

Comparison of the metabolic behavior of rat apolipoproteins A-I and A-IV, isolated from both lymph chylomicrons and serum high density lipoproteins

Rat apolipoprotein (apo) A-I and A-IV, isolated from both lymph chylomicrons and serum high density lipoproteins (HDL) were analyzed by isoelectric focusing. Lymph chylomicron apo A-I consisted for 81 +/- 2% of the pro form and for 19 +/- 2% of the mature form, while apo A-I isolated from serum HDL was present for 36 +/- 4% in the pro form and for 64 +/- 4% in the mature form. Apo A-IV also showed two major protein bands after analysis by isoelectric focusing. The most prominent component is the more basic protein that amounts to 80 +/- 2% in apo A-IV isolated from lymph chylomicrons and to 60 +/- 3% in apo A-IV isolated from serum HDL. Apo A-I (or apo A-IV), isolated from both sources (lymph chylomicrons or serum HDL), was iodinated and the radioactive apolipoproteins were incorporated into rat serum lipoproteins. The resulting labeled HDL was isolated from serum by molecular sieve chromatography on 6% agarose columns and injected intravenously into rats. No difference in the fractional turnover rate or the tissue uptake of the two labeled HDL preparations was observed, neither for apo A-I nor for apo A-IV. It is concluded that the physiological significance of the extracellular pro apo A-I conversion or the post-translational modification of apo A-IV is not related to the fractional turnover rate in serum or to the rate of catabolism in liver and kidneys.     

Increased beef consumption increases apolipoprotein A-I but not serum cholesterol of mildly hypercholesterolemic men with different levels of habitual beef intake

The objective of this research was to compare the effects of a lean beef enriched in oleic acid to a beef that is typical of the commercial beef consumed in the United States. Ten mildly hypercholesterolemic men, ages 34-58 years old, were selected from the Texas A&M University faculty and staff. Subjects were randomly assigned to one of two diets for a 6-week duration followed by a crossover after a 4-week habitual diet washout period. Diets were consumed daily for a 6-week study period. Participants substituted lean beef obtained from Wagyu bullocks or commercial beef for the meat typically consumed. Total cholesterol, apolipoproteins A-I and B, triacylglycerols, and low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol were measured in serum samples collected weekly. Beef type had no effect on any measured variable. There were no significant differences between baseline HDL or LDL cholesterol concentrations after the consumption of the beef test diets. Apolipoprotein A-I, serum glucose, and uric acid concentrations were elevated by the additional dietary beef. Analysis of records of customary diets indicated that one group consumed 160 g of beef daily, whereas the other group consumed only 26 g of beef daily. Therefore, post hoc analyses tested the habitual beef intake x treatment time interaction. LDL cholesterol concentration was markedly higher in the group with low habitual beef intake (180 vs 144 mg/dl), and HDL cholesterol was slightly higher (44 vs 40 mg/dl; post-test values) than for the group with high habitual beef intake, but there were no habitual intake x time interactions for LDL or HDL cholesterol. Creatinine and blood urea nitrogen concentrations also were greater in the individuals habitually consuming less beef. This study had three important findings: i) a lean beef source enriched with oleic acid was no different from commercial beef in its effect on lipoprotein fractions; ii) neither previous level of beef intake nor baseline LDL cholesterol concentration influenced the serum cholesterol response to added dietary beef, which was negative; and iii) apolipoprotein A-I, but not HDL or LDL cholesterol, was sensitive to the additional dietary beef.     

Human high density lipoprotein (HDL3) binding to rat liver plasma membranes

The binding of human 125I-labeled HDL3 to purified rat liver plasma membranes was studied. 125I-labeled HDL3 bound to the membranes with a dissociation constant of 10.5 micrograms protein/ml and a maximum binding of 3.45 micrograms protein/mg membrane protein. The 125I-labeled HDL3-binding activity was primarily associated with the plasma membrane fraction of the rat liver membranes. The amount of 125I-labeled HDL3 bound to the membranes was dependent on the temperature of incubation. The binding of 125I-labeled HDL3 to the rat liver plasma membranes was competitively inhibited by unlabeled human HDL3, rat HDL, HDL from nephrotic rats enriched in apolipoprotein A-I and phosphatidylcholine complexes of human apolipoprotein A-I, but not by human or rat LDL, free human apolipoprotein A-I or phosphatidylcholine vesicles. Human 125I-labeled apolipoprotein A-I complexed with egg phosphatidylcholine bound to rat liver plasma membranes with high affinity and saturability, and the binding constants were similar to those of human 125I-labeled HDL3. The 125I-labeled HDL3-binding activity of the membranes was not sensitive to pronase or phospholipase A2; however, prior treatment of the membranes with phospholipase A2 followed by pronase digestion resulted in loss of the binding activity. Heating the membranes at 100 degrees C for 30 min also resulted in an almost complete loss of the 125I-labeled HDL3-binding activity.     

A saturable, high-affinity binding site for human low density lipoprotein on freshly isolated rat hepatocytes

Freshly isolated rat hepatocytes bind the solely apolipoprotein B-containing human low density lipoprotein (LDL) with a high-affinity component. After 1 h of incubation less than 30% of the cell-associated human LDL is internalized and no evidence for any subsequent high-affinity degradation was obtained. Scatchard analysis of the binding data for human 125I-labeled LDL indicates that the high-affinity receptor for human LDL on rat hepatocytes possesses a Kd of 2.6 x 10(-8)M, while the binding is dependent on the extracellular Ca2+ concentration. Competition experiments indicate that both the apolipoprotein B-containing lipoproteins (human LDL and rat LDL) as well as the apolipoprotein E-containing lipoproteins (human HDL and rat HDL) do compete for the same surface receptor. It is concluded that hepatocytes freshly isolated from untreated rats do contain, in addition to the earlier described rat lipoprotein receptor which does not interact with human apolipoprotein B-containing LDL, a high-affinity receptor which interacts both with solely apolipoprotein B-containing human LDL and apolipoprotein E-containing lipoproteins.     

Characteristics of proton excretion in normal and acidotic toad urinary bladder

Leupeptin, an inhibitor of lysosomal cathepsin activity, was injected intravenously into male rats. Tissues obtained from leupeptin-treated animals showed a depressed cathepsin activity when compared with tissues from saline-treated control animals. Leupeptin treatment did not change the hepatic activities and subcellular distribution of marker enzymes for mitochondria, microsomes and plasma membranes. Hepatic lysosomal cathepsin activity was specifically inhibited, but the subcellular distribution of all lysosomal marker enzymes tested was changed, indicating the occurrence of enlarged lysosomes in the leupeptin-treated animals. No significant differences were observed in the serum concentrations of protein, cholesterol, cholesteryl esters, phospholipids and apolipoproteins A-I, A-IV and E between leupeptin-treated rats and control animals. When radioiodinated asialofetuin was injected intravenously, the radiolabel was retained for an extended period of time in the liver of leupeptin-treated animals, indicating diminished catabolism of this protein in the liver. When rat high-density lipoprotein, labelled specifically in the apolipoprotein A-I or E moiety was injected intravenously, only the kidneys and the liver showed a leupeptin-induced accumulation of radioactivity. These studies provide evidence for an important contribution of the kidneys and the liver to the in vivo catabolism of high-density lipoprotein apolipoproteins, using a method completely different from sugar-containing labelling compounds.     

Transfer of free and esterified cholesterol from low-density lipoproteins and high-density lipoproteins to human adipocytes

Cholesterol stored in human adipose tissue is derived from circulating lipoproteins. To delineate the cholesterol transport function of LDL and HDL, the movement of radiolabelled esterified cholesterol and free cholesterol from labelled LDL and HDL to human adipocytes was examined in the present study. LDL and HDL were enriched and labelled in esterified cholesterol with [14C]cholesterol by the action of plasma lipid transfer proteins and lecithin-cholesterol acyltransferase. Doubly labelled (3H,14C) LDL and HDL were prepared by exchanging free [3H]cholesterol into the 14C-labelled lipoproteins. 14C-labelled lipoprotein and 3H-labelled lipoprotein were also prepared separately and mixed to yield a mixed doubly labelled lipoprotein. Relative to the total amount added, proportionally more free than esterified cholesterol was transferred to the adipocytes upon incubation with any doubly labelled LDL and HDL. The calculated mass of free and esterified cholesterol transferred, however, varied with different labelled lipoproteins. 3H- and 14C-labelled LDL or HDL transferred 2-3-fold more esterified than free cholesterol while the reverse occurred with the mixed doubly labelled LDL or HDL. Thus, free cholesterol-depleted particles preferentially transferred cholesterol ester to the fat cells. In the presence of the homologous unlabelled native lipoprotein, the transfers of free and esterified cholesterol from labelled LDL or HDL were specifically inhibited. Selective transfer of esterified cholesterol relative to apoprotein was also observed when esterified cholesterol uptake from both LDL and HDL was assayed along with the binding of 125I-labelled lipoprotein. The cellular accumulation of cholesterol ether-labelled HDL (a non-hydrolyzable analogue of cholesterol ester) exceeded that of cholesterol ester consistent with significant hydrolysis of the latter physiological substrate. These results demonstrate preferential transfer of free cholesterol and esterified cholesterol over apoprotein for both LDL and HDL in human adipocytes. Furthermore, the data suggest that the cholesterol ester transport function of LDL and HDL can be enhanced by free cholesterol depletion and cholesterol ester enrichment of the particles, and affirms a role for adipose tissue in the metabolism of lipid-modified lipoproteins.     

Distribution of apolipoproteins A-I and A-IV among lipoprotein classes in rat mesenteric lymph, fractionated by molecular sieve chromatography

The distribution of apolipoproteins A-I and A-IV among lymph lipoprotein fractions was studied after separation by molecular sieve chromatography, avoiding any ultracentrifugation. Lymph was obtained from rats infused either with a glucose solution or with a triacylglycerol emulsion. Relative to glucose infusion, triacylglycerol infusion caused a 20-fold increase in the output of triacylglycerol, coupled with a 4-fold increase in output of apolipoprotein A-IV. The output of apolipoprotein A-I was only elevated 2-fold. Chromatography on 6% agarose showed that lymph apolipoproteins A-I and A-IV are present on triacylglycerol-rich particles and on particles of the size of HDL. In addition, apolipoprotein A-IV is also present as 'free' apolipoprotein A-IV. The increase in apolipoprotein A-I output is caused by a higher output of A-I associated with large chylomicrons only, while the increase in apolipoprotein A-IV output is reflected by an increased output in all lymph lipoprotein fractions, including lymph HDL and 'free' apolipoprotein A-IV. The increased level of 'free' A-IV, seen in fatty lymph, may contribute to, and at least partly explain, the high concentrations of 'free' apolipoprotein A-IV present in serum obtained from fed animals.     

Triiodothyronine increases rat apolipoprotein A-I synthesis and alters high-density lipoprotein composition in vivo

The effects of altered serum 3,3',5-triiodothyronine levels on rat lipoprotein metabolism were examined. Daily injections of the hormone (50 micrograms/100 g body mass) over a period of six days led to an increase of 6.4-fold in the hepatic mRNA level for apolipoprotein(apo)A-I, and a 21% increase in serum apoA-I levels. 12h after a single injection of 3,3',5-triiodothyronine the rate of [14C]leucine incorporation into apoA-I increased 2.1 fold. Conversely, in hypothyroid rats there was a decrease in hepatic mRNA levels for apoA-I and a decreased rate of [14C]leucine incorporation into apoA-I. The increase in hepatic apoA-I mRNA levels following 3,3',5-triiodothyronine treatment occurred prior to significant changes in serum triacylglycerol levels. High-density lipoprotein (HDL) particles isolated from the serum of hyperthyroid rats were smaller and enriched in apoA-I compared to apoA-IV and apoE. Similar changes in HDL composition were observed following in vitro incubations of normal rat serum with purified rat apoA-I. The results suggest that during altered thyroid status, changes in serum HDL size and composition occur in association with significant changes in apoA-I gene expression.     

High level expression of human apolipoprotein A-I in transgenic rats raises total serum high density lipoprotein cholesterol and lowers rat apolipoprotein A-I

To examine the consequences of increased apolipoprotein A-I production on cholesterol and lipoprotein metabolism, we have produced two lines of transgenic rats; one expressing moderate and one very high levels of human apolipoprotein A-I. The rats were produced by microinjection of a 13 kbp DNA fragment containing the human apolipoprotein A-I gene plus 10 kbp of its 5′ flanking sequence and 1 kbp of its 3′ flanking sequence. Both lines of transgenic rats express human apolipoprotein A-I mRNA in liver and human apolipoprotein A-I in plasma. Sera from these rats contain significantly higher levels of total apolipoprotein A-I, high density lipoprotein cholesterol and phospholipid than sera from non-transgenic littermates. Transgenic rats expressing high levels of human apolipoprotein A-I have reduced levels of serum rat apolipoprotein A-I suggesting a mechanism exists to down-regulate apolipoprotein A-I production. These transgenic rats provide a unique animal model to examine the effects of increased apolipoprotein A-I production on lipid and lipoprotein metabolism.     

Characterization of the specific binding of rat apolipoprotein E-deficient HDL to rat hepatic plasma membranes

We have used a preparation of rat liver plasma membranes to study the binding of rat apolipoprotein E-deficient HDL to rat liver. The membranes were found to bind HDL by a saturable process that was competed for by excess unlabeled HDL. The binding was temperature-dependent and was 85% receptor-mediated when incubated at 4, 22 and 37 degrees C. The affinity of the binding site for the HDL was consistent at all temperatures, while the maximum binding capacity increased at higher temperatures. The specific binding of HDL to the membranes did not require calcium and was independent of the concentration of NaCl in the media. The effect of varying the pH of the media on HDL binding was small, being 30% higher at pH 6.5 than at pH 9.0. Both rat HDL and human HDL3 were found to compete for the binding of rat HDL to the membranes, whereas rat VLDL remnants and human LDL did not compete. At 4 degrees C, complexes of dimyristoylphosphatidylcholine (DMPC) and apolipoproteins A-I, A-IV and the C apolipoproteins, but not apolipoprotein E, competed for HDL binding to the membranes. At 22 and 37 degrees C, all DMPC-apolipoprotein complexes competed to a similar extent, DMPC vesicles that contained no protein did not compete for the binding of HDL. These results suggest that the rat liver possesses a specific receptor for apolipoprotein E-deficient HDL that recognizes apolipoproteins A-I, A-IV and the C apolipoproteins as ligands.     

Changes in the concentration of plasma lipoproteins and apoproteins following the administration of Triton WR 1339 to rats.

Changes in whole plasma and lipoprotien apoprotein concentrations were determined after a single injection of Triton WR 1339 into rats. Concentrations of apoproteins A-I (an activator of lecithin:cholesterol acyl transferase), arginine-rich apoprotein (ARP), and B apoprotein were measured by electroimmunoassay. The content of C-II apoprotein (an activaor of lipoprotein lipase) was estimated by the ability of plasma and lipoprotein fractions to promote hydrolysis of triglyceride in the presence of cow's milk lipase and also by isoelectric focusing on polyacrylamide gels. Apoproteins C-II and A-I were rapidly removed from high density lipoprotein (HDL) after Triton treatment and were recovered in the d 1.21 g/ml infranate fraction. A-I was then totally cleared from the plasma within 10--20 hr after injection. Arginine-rich apoprotein was removed from HDL and also partially cleared from the plasma. The rise in very low density lipoprotein (vldl) apoprotein that followed the removal of apoproteins from HDL was mostly antributed to the B apoprotein, although corresponding smaller increases were observed in VLDL ARP and C apoproteins. The triglyceride:cholesterol, triglyceride:protein, and B:C apoprotein ratios of VLDL more closely resembled nascent rather than plasma VLDL 10 hr after Triton injection. These studies suggest that the detergent may achieve its hyperlipidemic effct by disrupting HDL and thus removing the A-I and C-II proteins from a normal activating environment compirsing VLDL, HDL, and the enzymes. The possible involvement of intact HDL in VLDL catabolism is discussed in relation to other recent reports which also suggest that abnormalities of the VLDL-LDL system may be due to the absence of normal HDL.     

Quantitative determination of apolipoprotein A-I in high-density lipoproteins and 'free' apolipoprotein A-I by two-dimensional agarose gel lipoprotein-'rocket' immunoelectrophoresis of human serum

A method has been developed for quantitative analysis of 'free' apolipoprotein A-I and apolipoprotein A-I associated with high-density lipoprotein (HDL) in serum. The method utilizes the difference between the rate of electrophoretic migration of apolipoprotein A-I associated with HDL (alpha) and 'free' apolipoprotein A-I (pre-beta) in agarose gel. Apolipoprotein A-I is subsequently quantitated by electrophoresis in a second dimensional gel containing anti-apolipoprotein A-I antibodies. Using this method all apolipoprotein A-I of normal fasting serum was found associated with HDL (n = 16). By contrast, 'free' apolipoprotein A-I accounted for up to 12% of the total in the serum of patients with isolated hypertriglyceridemia (n = 8) or mixed hyperlipoproteinemia (n = 8). Between 30 and 35% of 'free' apolipoprotein A-I was found in one patient afflicted with the apolipoprotein C-II deficiency syndrome. Also, 'free' apolipoprotein A-I could be detected in normal postabsorptive serum. 30 and 90 min following heparin-enhanced lipolysis 'free' apolipoprotein A-I accounted for 23 and 20%, respectively, of the total apolipoprotein A-I of serum. Apolipoprotein A-I associated with HDL remained unaltered. It appears, therefore, that 'free' apolipoprotein A-I is liberated from triglyceride-rich lipoproteins during lipolysis.     

Human apolipoprotein A-I forms small high density lipoprotein particles in rats in vivo.

The effects of injection of purified human or rat apolipoprotein (apo) A-I (1.7 mg/100 g body weight) on the size and composition of rat high density lipoprotein (HDL) particles have been investigated. The injection of human apo A-I results in the formation (over a period of 3 to 6 h) of a population of smaller HDL particles resembling human HDL3. This population of smaller particles contains human apo A-I and rat apo A-IV but lacks rat apo A-I and rat apo E. Small HDL3-like particles are not detected in rat plasma following the injection of rat apo A-I. Associated with the injection of either human or rat apo A-I is a gradual increase of plasma cholesterol levels of 20 to 50% (over 24 h) and the appearance of larger HDL particles. The results suggest that the smaller HDL particles in human plasma compared to rat plasma are not simply due to the action of lipid modifying enzymes or lipid transfer proteins but a specific property of human apo A-I.     

Application of O-(4-diazo-3-[125I]iodobenzoyl)sucrose for the detection of the catabolic sites of low density lipoprotein

The sites of degradation of human low density lipoprotein (LDL), are analyzed using the novel labelling compound O-(4-diazo-3-[125I]iodobenzoyl)sucrose (D125IBS). The decay from rat serum of D125IBS-labelled LDL is identical to the serum decay of conventionally iodinated (ICI method) LDL. The radioactivity derived from D125IBS-labelled LDL accumulates predominantly in the liver after intravenous injection and the hepatic radioactivity remains associated with the lysosomal compartment for an extended period of time, when compared to the radioactivity derived from conventionally iodinated LDL. It is concluded that the D125IBS labelling procedure is an interesting new tool to study the sites of catabolism of serum lipoproteins.     

Effects of probucol on plasma lipids, lipoproteins and parameters of high density lipoprotein metabolism.

Eight patients with primary hypercholesterolemia were treated with probucol for 17 weeks. Plasma total cholesterol, low density lipoprotein (LDL)-cholesterol, and high density lipoprotein (HDL)-cholesterol decreased by 16.6, 15.0 and 25.7%, respectively, in response to probucol treatment. Plasma levels of apolipoprotein B and apolipoprotein A-I also decreased, while apolipoprotein A-II concentrations were unchanged. The decrease in HDL-cholesterol levels was associated with a reduction in HDL particle size. No changes in the plasma lecithin:cholesterol acyltransferase activity or mass occurred in response to probucol treatment. In contrast, a significant 25% increase in plasma cholesteryl ester and triglyceride transfer activity occurred following probucol treatment. There was a positive correlation (R = 0.94) between cholesterol ester and triglyceride transfer. We propose that the increase in lipid transfer activity may in part explain the changes in HDL concentration and size, as well as the previously reported effect probucol has on reducing atherosclerosis in animal models.